In the race to mitigate the negative impacts of the rapidly advancing climate crisis, it is evident that reliable ways to remove carbon dioxide from the atmosphere are needed in addition to emission cuts. One emerging carbon dioxide removal (CDR) approach is Marine Anoxic Carbon Storage (MACS). The key concept of MACS is simple: depositing land-derived (terrestrial) biomass to lock carbon away in deep, anoxic (= oxygen-depleted) marine basins, where it can remain for centuries or longer. The science behind MACS is promising, but as with any new climate solution, the details and risk profiles matter, and so the key question is not just “Could this work?”, but rather “Can we do it safely, responsibly and at scale?”.

A new scientific review led by Dr. Morgan Reed Raven takes a structured look at this question by summarizing the state of knowledge in order to pave the way for addressing risks and knowledge gaps that could limit MACS deployment in places like the Black Sea and deep brine reservoirs including the Orca Basin in the Gulf of Mexico. At the same time, the recently released Puro.earth MACS methodology sets the world’s first carbon-accounting framework for MACS, and already includes safeguards aligned with many of the risks raised by the review paper.

Together, these two documents paint a clearer picture of what responsible MACS development should look like.

What could limit safe and effective deployment of MACS on a global scale? 

1. Long-term storage durability: 
   Changes in ocean mixing or circulation could transport breakdown products into oxygenated waters, where stored carbon might eventually escape as CO2. Overfilling a basin could also reduce storage capacity.
2. Biomass availability and sustainability: 
   MACS requires large amounts of sustainable, economically viable biomass that can be transported efficiently at scale to accessible storage sites. Thus, one challenge for responsible MACS deployment is to avoid negative consequences from unsustainable diversion of biomass to the ocean: large emissions from biomass transport, land-use conflicts, negative effects on soil health, or decreased agricultural productivity. 
3. Greenhouse gas balance: 
   To achieve net greenhouse gas removal, MACS activities will need to minimize the release of CO2, methane, or nitrous oxide from transportation, biomass breakdown, or the disturbance of methane hydrates or brine interfaces on the seafloor.
4. Preservation of the oxygenated part of the marine basins: 
   Harmful materials like sulfide are possible by-products of decomposing terrestrial biomass in anoxic basins. Responsible MACS deployments must monitor and prevent the contact of these materials with oxygenated layers of the marine basins in order to avoid oxygen depletion and negative impacts on marine life.
5. Nutrient or organic matter impacts in the oxic zone: 
   Decomposition of biomass could also result in additional nutrients or dissolved organic matter production. If those reach near-surface waters, they might fuel algal blooms.

Each of these important topics represents a potential practical limitation to the responsible implementation of MACS at scale, as well as an important area for focused research. MACS must be developed with a clear focus on environmental, social and operational safeguards and robust monitoring framework.

The core aim of the paper was to identify the criteria that could limit how far MACS can scale, and to determine which ones represent the highest priorities for research, monitoring and engineering design. In doing so, the authors provide one of the clearest roadmaps yet for evaluating MACS as a future carbon removal pathway. The lead author Dr. Raven says:

The key message of the paper is pragmatic: MACS has real potential, but only if we understand and navigate the remaining constraints. With careful science and transparent risk assessment, MACS has large potential to become an important part of a broader climate solution. But getting it right requires asking the hard questions now.

How Puro’s MACS methodology already addresses key risks

Many of these concerns are already incorporated into the MACS methodology requirements. From early on, MACS was developed in close collaboration with researchers, MACS project developers and other relevant stakeholders. While the science is still evolving, the methodology already includes robust safeguards to ensure reliable and safe science-based carbon removals. Examples of such criteria include:

1. Strict biomass eligibility criteria
   To minimise nutrient release or contamination in anoxic waters, only terrestrial biomass with physical and chemical composition that makes it resistant to decomposition is eligible. 
2. Robust site selection and environmental assessment
   Eligible storage sites are limited to stable anoxic basins, where the circulation dynamics and stratification of the water masses limit the transport of biomass decomposition products to the oxic surface waters and to the atmosphere.
3. Comprehensive monitoring, reporting and verification
   Comprehensive monitoring activities before, during, and after biomass deployment that are based on scientifically robust methodology are required. 
4. Adaptive and science-responsive governance
   Each project must meet relevant local, regional, national or international permitting requirements. The methodology is also designed to evolve as new science emerges, ensuring that the applicable safeguards are re-assessed when necessary.

A path forward: Combining innovation with responsibility

MACS represents one of the more creative and potentially scalable carbon storage ideas on the horizon. It is also unique in the sense that it stands in the crossroads between terrestrial and marine CDR approaches. The new scientific review makes clear that MACS comes with real challenges and constraints, but it also identifies where those constraints are likely to emerge and how to mitigate them.

The Puro.earth MACS methodology complements this by setting the necessary safeguards. Together, they set the basis for a future in which MACS is deployed responsibly, based on scientific understanding, and in a way that protects marine ecosystems while contributing meaningfully towards climate mitigation.

A combination of high-quality scientific research and rigorous standards will be needed if MACS is to feature prominently in the climate solution portfolio.